He was so good at dodging the cameras that the only known shot of him was one taken in 2015 by a Fairfax photographer who staked out his home in Sydney's eastern suburbs for days.

But on Wednesday September 11, he could no longer avoid the spotlight.

The media pack surrounded Mr Teoh for the rare photo opportunity, as he walked into Melbourne's Federal Court to give evidence in a case that will decide the future shape of Australia's mobile telecommunications networks.

It has been a fascinating insight into the mind of Mr Teoh, and the company, which he founded with his wife Vicky in 1992 after emigrating from Malaysia in 1986.

Why the ACCC blocked the TPG-Vodafone merger

The key contention being tested in court is whether Australia's competition watchdog was right to block a $15-billion merger between TPG and Vodafone Hutchison.

The Australian Competition and Consumer Commission (ACCC) had blocked the merger on the grounds it would lessen competition in the sector currently dominated by Telstra, Optus and Vodafone.

They are the three main companies competing in the mobile network market, while Telstra, Optus and TPG — which owns the iiNet and internode brands — are the three largest players in the broadband market.

In her opening statement on Tuesday, the first day of the three-week trial, TPG counsel Ruth Higgins SC said the ACCC's case was built on a "fallacy".

Not only can TPG not afford to build Australia's fourth mobile network, Dr Higgins argued, but it does not want to.

"The ACCC has said to TPG 'build it and they will come'," Dr Higgins said.

"TPG did try to build it, but it was thwarted by community objections, by technical difficulties but, ultimately, by the Federal Government's security guidance."

Vodafone's lawyer, Peter Brereton, said in his opening statement it would be "commercially crazy" for TPG to build its own mobile network to rival Vodafone's and argued the ACCC's case was based on "speculation and possibilities".

"The notion that TPG would, if the merger's blocked, roll out a mobile network is just not of the real commercial world," Mr Brereton said.

But counsel for the ACCC Michael Hodge QC — known for firing hard questions at AMP during the banking royal commission — told the court, "It is entirely commercially realistic that TPG will return to rolling out a mobile network".

Vodafone Australia boss Inaki Berroeta spent much of Tuesday afternoon detailing the challenging market conditions the company currently faced, even going so far as suggesting an outlook statement from the Vodafone annual report issued in December was too optimistic.